The present invention concerns an adaptive hybrid method for access to a transmission channel.
A centralised network has a base station (also referred to as the xe2x80x9ccentral stationxe2x80x9d) and mobile stations (also referred to as xe2x80x9cperipheral stationsxe2x80x9d) which are supervised by the base stations, that is to say they can access the network only when the base station gives them authorisation to do so, by allocating to them individually or collectively a momentary access right or xe2x80x9ctokenxe2x80x9d, corresponding to an interval of time during which one or more stations are authorised to send.
Two principal access methods are known. The first, referred to as xe2x80x9cpollingxe2x80x9d, consists of successively allocating individual access rights which authorise one of the mobile stations to access the network. The second, referred to as xe2x80x9ccontentionxe2x80x9d, consists of allocating a right of access simultaneously to all the mobile stations. In this second case, each mobile station is authorised to access the network at any moment, according to procedures which depend only on its internal functioning, and risks of collision arise.
Two performance criteria for the networks which are generally used are on the one hand the maximum total throughput of data circulating therein and on the other hand the mean latency, that is to say the mean period which elapses between the instant when a frame is ready to be sent and the correct and complete reception of this frame.
It will easily be understood that the polling method makes it possible to achieve a very high theoretical total throughput but entails a high latency, whereas the contention method affords low latency but does not allow a very high maximum throughput.
The polling method is in fact best suited to cases where the total throughput required for all transmissions of information between the stations is high, whereas the contention method is most suited to low throughputs relating to an available passband.
The document U.S. Pat. No. 4,071,908 (Bell Telephone Lab) describes a hybrid access method which uses the principle of a progressive subdivision of a group which initially contains all the stations, in making the division account being taken only of the criterion of the total transmission load on the channel. Each division takes place in groups, the composition of which is predetermined by the addresses of the stations, as long as a group replies to a token and includes at least two stations. Thus, in many cases, stations which have only a small requirement for access to the channel are allocated individual access rights, because of the proximity of their address to those of the stations which have a high requirement for access to the channel. Access to the channel is then poorly distributed between the stations.
The document WO 91/16775 presents a hybrid communication system which allocates individual access rights according to the number of collisions which is detected. The groups are, there too, defined by the addresses of the stations. This system has the same drawbacks as the one described in the document U.S. Pat. No. 4,071,908.
The present invention sets out to remedy these drawbacks. In particular, the invention is distinguished by the determination of the requirement of each station for access to the channel and then by the determination of groups according to these access requirements, and then by allocating access rights to the stations in each group, in succession.
It should be noted here that the access requirements are preferentially measured either in proportion to the distributed time intervals, or in proportion to the period of use of the shared channel.
To this end, the present invention relates, according to a first aspect, to a method for access to a transmission channel shared between several stations, characterised in that it includes iteratively:
an operation of estimating requirements of each of the said stations for access to the transmission channel,
an operation of determining groups of stations taking into account the estimated access requirement of the said stations, at least one of the said groups of stations being said to be xe2x80x9ccollectivexe2x80x9d,
an operation of allocating, to each of the said groups, access rights each corresponding to an interval of time during which the stations which do not form part of the said group are not authorised to access the transmission channel, and during which, if the said group is collective, each station which it includes gains access to the transmission channel according to a contention method.
It should be noted here that an estimation of the access requirement of a station amounts to defining an estimated access requirement which represents the true access requirement of the said station. For example, in order to make the said estimation, it is possible to take account of the number of accesses already effected by the said station, the number of collisions suffered by the data frames sent by the said station, the number of frames remaining to be sent by the said station or the quality of service required by the said station.
By virtue of these provisions, each station, whose access requirement is first estimated, is associated with a group according to this access requirement. The invention therefore makes it possible to allocate the access rights progressively according to the estimated access requirement of each station.
According to particular characteristics, during the operation of determining groups, at most only one group having more than one station is determined.
This group is referred to as xe2x80x9ccollectivexe2x80x9d in distinction to the groups which contain only one station and which are hereinafter called xe2x80x9cindividualxe2x80x9d.
By virtue of these provisions, apart from the stations in the said collective group, all the other stations can receive individual rights of access to the transmission channel. In addition the presence of each station in any collective group depends only on their estimated access requirement. Finally, the stations in the said collective group can be allocated collective access rights. The type, individual or collective, of the access rights allocated to a station is then a function of the estimated access requirement of this station.
According to particular characteristics, the access method as briefly disclosed above:
includes an operation of defining a first so-called xe2x80x9cstabilityxe2x80x9d threshold applicable to the sum of the estimated access requirements of a group, and
during the group determination operation, each group is determined so that no collective group has a sum of access requirements of the stations which it includes which is greater than the said first stability threshold.
Preferentially, the determination of the said first stability threshold takes account of the contention method concerned.
By virtue of these provisions, when collective access rights are given to the stations in the said collective group, the stability of the network can be guaranteed, since the estimation of the access requirements is reliable.
According to particular characteristics, the group determination operation includes:
a so-called xe2x80x9cstabilityxe2x80x9d test during which it is determined whether the sum of the estimated access requirements of all the stations is greater than the said first stability threshold, and
in the affirmative, an operation of generating at least one individual group each including one and only one of the stations which have an estimated access requirement greater than or equal to the estimated access requirement of each other station.
By virtue of these provisions, when access by contention for all the stations would lead to saturation of the channel, at least two groups are determined, some of which, individual, each have a station amongst those whose estimated access requirement is the highest, and the last, generally collective, includes all the other stations. It is then possible to allocate to the first ones an individual access right and to the last one a collective access right.
According to particular characteristics, conjointly with the implementation of the said generation operation, the group determination operation includes an operation of defining a second so-called xe2x80x9cstabilityxe2x80x9d threshold and iteratively:
a so-called xe2x80x9cstabilityxe2x80x9d test during which it is determined whether the sum Pxe2x80x2 of the estimated access requirements of all the stations which are not already in an individual group, relative to the available passband for these stations, is greater than the said second stability threshold Sxe2x80x2, and
in the affirmative, a new operation of generating at least one individual group each including one and one only of the stations which have an estimated access requirement greater than or equal to the estimated access requirement of each other station which is not already in an individual group.
By virtue of these provisions, the individual groups are iteratively constituted so that the stations which remain and which, collectively, do not have a total access requirement which risks saturating the channel, may, conjointly, be allocated a collective access right, whilst the other stations are each allocated an individual group which can receive an individual access right.
According to particular characteristics, during the group determination operation, when a station is in an individual group, each station whose estimated access requirement is strictly greater than the estimated access requirement of the said station, is in an individual group.
By virtue of these provisions, all the stations which have an access requirement which is estimated to be greater than a given value, are in an individual group. Thus it is possible to allocate to each of them an individual access right, whilst the stations which have a lesser requirement for access to the channel are grouped together in a collective group to which collective access rights can be allocated.
According to particular characteristics, during the group determination operation, when a station is in an individual group, each station whose estimated access requirement is greater than or equal to the estimated access requirement of the said station, is in an individual group.
By virtue of these provisions, two stations which have the same estimated access requirement are always in the same situation, either, together, in the collective group, or, each, in an individual group.
According to particular characteristics:
during the access requirement estimation operation, the access requirement of each station is estimated by taking into account at least a number of accesses effected previously by the said station,
during the access requirement estimation operation, the access requirement of each station is estimated by taking into account at least one estimation of a number of collisions previously suffered by data frames sent by the said station, and/or
during the access requirement estimation operation, the access requirement of each station is estimated by taking into account at least one estimation of a number of accesses remaining to be effected by the said station.
By virtue of each of these provisions, it is easy to estimate access requirements.
It should be noted that, by taking into account several numbers representing for example the accesses effected, the collisions or the accesses remaining to be effected, it is possible to estimate access requirements by taking account of an interpolation of the said numbers.
According to particular characteristics, during the access requirement estimation operation, the access requirement of each station is estimated by taking into account the service quality constraints required for access of the said station.
By virtue of these provisions, the contract on which the service quality is based can be complied with, that is to say it is aimed to guarantee, amongst other things, a latency and a quantity of access per unit time for certain stations.
According to particular characteristics, during the access right allocation operation, there is allocated to each individual group a proportion of all access rights which is an increasing function of the access requirement estimations for the said group.
By virtue of these provisions, the allocation of the access rights is intended to absorb the entire access requirement of each individual group.
According to particular characteristics:
during the access right allocation operation, the total number of access rights allocated is a constant value between the successive access right allocation operations, and/or
during the access right allocation operation, the total duration of all the access rights allocated is a constant value.
By virtue of these provisions, the frequency of interrogation of each station is greater than or equal to a predetermined value, which can be useful in the case of isochronous flows, such as those concerning the transmission of speech and/or images, with a quality of service which is simple to implement.
According to a second aspect, which concerns particularly a peripheral station, the present invention relates to a method of access to a transmission channel, for a given station, characterised in that it includes:
an operation of estimating a requirement for access to the said channel for the said station,
an operation of receiving access rights,
for collective access rights:
an operation of determining a threshold access requirement value, taking into account an item of information representing a threshold access requirement in the said collective access right,
a test during which it is determined whether the estimated access requirement is less than the said threshold access requirement, and
in the event of a positive result, an access to the said channel according to a contention access method.
According to a third aspect, which concerns more particularly a peripheral station, the present invention relates to a method of access, for a given station, to a transmission channel shared between several stations, characterised in that it includes:
an operation of estimating requirements for access to the transmission channel of each of the said stations,
an operation of determining groups of stations each including at least one station, the said operation taking into account the estimated access requirement of the said stations,
an operation of receiving access rights
for collective access rights:
an operation of determining a threshold access requirement value, taking into account an item of information representing a threshold access requirement in the said collective access right,
a test during which it is determined whether the estimated access requirement is less than the said threshold access requirement, and
in the event of a positive result, an access to the said channel according to a contention access method.
Since the advantages of the second and third aspects of the invention are the same as those of the first aspect, they are not repeated here.
According to a fourth aspect, the present invention relates to a device for access to a transmission channel shared between several stations, characterised in that it includes processing means adapted to, iteratively:
estimate requirements of each of the said stations for access to the transmission channel,
determine groups of stations taking into account the estimated access requirement of the said stations, at least one of the said groups of stations being said to be xe2x80x9ccollectivexe2x80x9d,
allocate, to each of the said groups, access rights each corresponding to an interval of time during which the stations which do not form part of the said group are not authorised to access the transmission channel, and during which, if the said group is collective, each station which it includes gains access to the transmission channel according to a contention method.
According to a fifth aspect, the present invention relates to a device for access to a transmission channel, for a given station, characterised in that it includes processing means adapted to, iteratively:
estimate a requirement for access to the said channel for the said station,
receive access rights,
for collective access rights:
determine a threshold access requirement value, taking into account an item of information representing a threshold access requirement in the said collective access right,
determine whether the estimated access requirement is less than the said threshold access requirement, and
in the event of a positive result, and if an access must be effected by the said station, access the said channel according to a contention access method.
According to a sixth aspect, the present invention relates to a device for access, for a given station, to a transmission channel shared between several stations, characterised in that it has processing means adapted to, iteratively:
estimate a requirement for each of the said stations for access to the transmission channel,
determine groups of stations each including at least one station, taking into account the estimated access requirement of the said stations,
receive access rights,
for collective access rights:
determine a threshold access requirement value, taking into account an item of information representing a threshold access requirement in the said collective access right,
determine whether the estimated access requirement is less than the said threshold access requirement, and
in the event of a positive result, and if an access must be effected by the said station, access the said channel according to a contention access method.
Thus, according to the fifth and sixth aspects of the invention, each peripheral station receives a collective access right which includes an item of information representing a threshold or limit value. In order to determine whether or not this peripheral station is authorised to send, the latter compares its estimated access requirement, which, preferentially, the peripheral station determines in the same way as the central station, with the threshold value. When its estimated access requirement is above the threshold value, the peripheral station is not authorised to send. When its estimated access requirement is below the threshold value, the peripheral station is authorised to send according to a contention access method.
The invention also relates to a transmitter, a network master station, a network base station, a network peripheral station, a telephone, a facsimile machine, a network, notably a wireless network, and a bus, characterised in that they implement the method of the invention as briefly disclosed above. As these devices have the same advantages as the method which they implement, they are not restated here.